Yep it should be possible to retain your phone number when you change from ntl to BT, may take a little while to sort out though, but no doubt BT will try and sort it for you.
As for broadband, make sure when you get the new BT line setup, BT know you will want Broadband on it. I've heard that sometimes they "cheat" in adding lines and split them - phone works OK but the split line couldn't support the ADSL.
As for ADSL ISPs you might get a reasonable comparision on sites like
www.adslguide.org or
www.ispreview.co.uk. There are so many names to choose from.
Don't just look for the cheapest price. Do you want any add ons like webspace, email addresses, etc. Do you download a lot of stuff so could caps or throttling of connections be an issue? Do you want an ISP that gives you loads of added content such as the Yahoo, or AOL things, or woudl you be happy with just googling for the stuff you want? Do you want antispam, antivirus and such like cheap or bundled? Watch out for AOL if you don't want their proprietary email and browser software.